Integrated circuit assembly with optically coupled components

ABSTRACT

A system for optically coupling components of one or more integrated circuits comprising a light source, means for focussing the light onto a modulatable electro-optic material located on or in a substrate, the electro-optic material being disposed above and partially covering a reflective coating. The focussing means is arranged relative to the covered and uncovered portions of the reflective coating and relative to a detector element on or in the substrate whereby reflected modulated light from the electro-optic material and reflected unmodulated light from the uncovered portion of the reflective coating are superimposed to create an amplitude modulated beam for focussing on the detector element.

BACKGROUND OF THE INVENTION

The present invention relates to integrated circuits and moreparticularly to a method of optically coupling components of integratedcircuits.

DESCRIPTION OF THE RELATED ART

As is well known integrated circuits are complete electronic circuitscontaining transistors and perhaps diodes, resistors or capacitors allmade from, and on, a chip of semiconductor material, typically silicon.The connections between the components of the chip are electrical and inthe main confined to a two dimensional surface. When it is desired toprovide sets of electrical connections between components of large-scale(LSI) or very-large scale integrated (VLSI) circuit design difficultiesmay arise as a result of the lack of surface area on the chip. Eachintegrated circuit is normally packaged within its own protectiveplastic case, the packaging taking many different forms but all beingprovided with terminal pins for receiving and transmitting data. In LSIor VLSI circuits the number of terminal pins may be large compared withthe surface area of the chip and the terminal pins and theirinterconnections may give rise to circuit design difficulties which mayaffect the dependability of the circuits. The circuit design may befurther complicated when a number of integrated circuits areinterconnected to operate together as an integrated system as the numberof connections between the integrated circuits may be very large indeed.

SUMMARY OF THE INVENTION

According to the present invention, by providing a optical couplingbetween a modulator elements and a detector element on the chip theabove design difficulties can be largely overcome. Furthermore, anoptical coupling arrangement according to the present invention providesa method for coupling components on separate chips and provides a meansfor the electrical switching of components leading to a programmabledesign architecture for the chips.

According to the present invention there is provided a method of opticalcoupling between components of one or more integrated circuitscomprising the steps of focussing electromagnetic radiation onto amodulator element associated with the or one of the integrated circuits,modulating the radiation at the modulator element, reflecting themodulated radiation in a direction away from the modulator element andfocussing the modulated radiation onto a detector element associatedwith the same or a different integrated circuit.

An aspect of the present invention is the provision of a system foroptically coupling components of one or more integrated circuitscomprising a source of electromagnetic radiation, focussing means forfocussing the radiation from the source onto a modulator componentassociated with the or one of the integrated circuits, a reflectivemeans for reflecting the modulated radiation towards the focussingmeans, and a detector element associated with the same or a differentintegrated circuit for receiving the modulated radiation from thefocussing means.

Conveniently the electromagnetic radiation is light, that is to sayhaving a wavelength within that part of the spectrum between theinfra-red to ultra-violet. Advantageously a laser is used as the sourceof light and conveniently the wavelength of the light is substantiallywithin the range 0.6 to 10.0 μm.

Preferably the integrated circuit has a silicon substrate at least someof its component elements being formed by doping with group III or grouplV impurities.

In one embodiment of the present invention the detector element is a PINdiode formed in the substrate, the PIN diode being responsive tovariations in the intensity of the modulated radiation incident on thePIN diode.

In a preferred embodiment of the present invention the modulator elementis an electro-optic material applied to, formed in, or sputtered ontothe substrate, a pair of electrodes being disposed in or attached to thesubstrate in the vicinity of the electro-optic material for creating anelectric field in the electro-optic material when a voltage is appliedacross the electrodes. The electro-optic material and pair of electrodesform an electro-optic phase modulator to phase modulate a beam of lightpassing through the electro-optic material when the electric field isapplied. In the preferred embodiment a beam of light whose phase isunmodulated is superimposed on the phase modulated beam so as to createan amplitude modulated beam which is focussed on the detector element.

BRIEF DESCRIPTION OF THE PRIOR ART

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 shows in schematic form an optical coupling arrangement for anintegrated circuit in accordance with one embodiment of the presentinvention;

FIG. 2 is a top view of the modulator in FIG. 1; and

FIG. 3 shows in schematic form an optical coupling arrangement for anintegrated circuit in accordance with a second embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 a laser beam, from a source S, is focussed by apartially reflecting mirror 2 and a lens 4 onto a transparentelectro-optic material 6, such as PLZT, sputtered onto a reflectivecoating 8, typically formed on the surface of a silicon substrate 10.

Two spaced electrodes 12 are disposed in the substrate 10 below thematerial 6 so as to form therewith an electro-optic modulator. Inoperation a steady or varying voltage is applied between the electrodes12, the voltage being dependent on the signal which it is desired totransmit to a detector 14, typically a PIN diode, realised in thesilicon 10. The voltage across the electrodes 12 creates an electricfield within the electro-optic material 6 and by means of theelectro-optic effect produces a change in the refractive index of theelectro-optic material 6 as a function of the electric field strength.

The light wave as it passes through the electro-optic material 6experiences a phase change which is dependent on the change in therefractive index and on the length of the optical path of the laser beamthrough the electro-optic material.

The laser beam is reflected by the coating 8 back through theelectro-optic material 6, the phase modulated beam after leaving theelectro-optic material 6 being re-focussed by the lens 4 and thepartially reflecting mirror 2 onto the detector 14.

It is necessary to provide an amplitude modulated beam the intensity ofwhich can be detected by the detector 14. One method of providing anamplitude modulated laser beam at the detector 14 is to superimpose ontothe phase modulated beam the unmodulated laser beam from the source S soas to create a system which behaves as an interferometer. One suchsystem is shown in FIGS. 1 and 2 in which the reflective coating 8extends beyond the edges of the electro-optic material 6.

In operation the unmodulated laser beam is focussed by the lens 4 notonly on the electro-optic material 6 but also on the uncovered areas ofthe reflective coating 8. The portion of the laser beam reflected fromthe uncovered areas of the reflective coating 8 is unchanged in phaseand when re-focussed by the lens 4 and the mirror 2 arrangement onto thedetector 14 combines with the phase modulated beam. The system thereforebehaves as an interferometer and the output of the detector 14 varies asthe voltage-controlled phase-shift varies the degree of constructiveinterference.

An embodiment of the present invention will now be described withreference to FIG. 3, identical features to FIG. 1 being designated byidentical reference numerals.

The arrangement illustrated in FIG. 3 differs from FIG. 1 firstly inthat the substrate 10 has two modulator elements, the second beingdefined by an electro-optic material 8' and a pair of associatedelectrodes 12'. Similarly there is a second detector element in the formof a PIN diode 14'.

The second difference to the embodiment of FIG. 1 is the provision of awaveguide crossover electro-optic modulator 19. The modulator 19comprises an opaque block of material 20 having a number of waveguides,four of which in FIG. 3 are numbered 22, 24, 26, 28, extending betweenthe major faces of the block. The waveguides form optical paths throughthe block 20 and are arranged to define crossover points A, B, C, D. Thewaveguides, are made of an electro-optic material and each crossoverpoint A, B, C, D is associated with a respective pair of electrodes (notshown). In operation in the absence of an applied voltage the lightpassing along any particular waveguide passes straight through thecrossover point and out of the other end of the waveguide. On the otherhand the application of a voltage to a pair of electrodes reduces therefractive index of the material at the associated crossover point,through the electro-optic effect, sufficiently for total internalreflection to occur. This results in the light incident on the end ofone waveguide, for example the end of waveguide 22 located at one faceof the block 20, crossing over at point A to emerge from the end ofwaveguide 24 at the opposite face of the block 20. In this way themodulator 19 functions as a crossover switch so that light modulatedwithin the electro-optic material 6 and reflected can be focussed bymeans of the lens 4, modulator 19 and partially reflecting mirror 2 oneither or both of the PIN diodes 14, 14'. Likewise light modulated andreflected from the electro-optic material can be focussed on one or bothof the detectors 14, 14'.

It will be appreciated that the reflective element although described inthe embodiments above as a partially reflecting mirror 2 can be replacedby an arrangement of mirrors or a reflective hologram, the hologramgiving a general repeated pattern of connections. The use of amodulatable hologram provides an embodiment having switching functions.The retroreflective nature of the systems described above with ahologram or specific mirror arrangements also reduces alignmenttolerances.

In other embodiments of the present invention optical coupling isprovided between two or more chips side-by-side or (using two lenses)face-to-face.

I claim:
 1. A system for optically coupling components of one or moreintegrated circuits comprising a source of electromagnetic radiation,focussing means for focussing the radiation from the source onto amodulator component associated one of the integrated circuits, whereinthe modulator element is an electro-optic material applied to, formedin, or sputtered onto the substrate, a pair of electrodes being disposedin or attached to the substrate in the vicinity of the electro-opticmaterial for creating an electric field in the electro-optic materialwhen a voltage is applied across the electrodes, the electro-opticmaterial and pair of electrodes forming an electro-optic phase modulatorfor phase modulating a beam of light passing through the electro-opticmaterial when the electric field is applied, a reflective means locatednext to said modulator component for reflecting the resulting modulatedradiation towards the focussing means, and a detector element associatedwith one of said integrated circuits for receiving the modulatedradiation from the focussing means.
 2. A system for optically couplingcomponents of one or more integrated circuits as claimed in claim 1wherein the source of electromagnetic radiation is a laser fortransmitting a wavelength of light substantially within the range 0.6 to10 μm.
 3. A system for optically coupling components of one or moreintegrated circuits as claimed in claim 1 wherein the integrated circuithas a silicon substrate at least some of its component elements beingformed by doping with group III or group IV impurities.
 4. A system foroptically coupling components of one or more integrated circuits asclaimed in claim 1 wherein the detector element is a PIN diode formed inthe substrate, the PIN diode being responsive to variations in theintensity of the modulated radiation incident on the PIN diode.
 5. Asystem for optically coupling components of one or more integratedcircuits as claimed in claim 1 wherein the reflective means and thefocussing means are arranged relative to one another, and relative tothe detector element and the modulator component to superimpose a beamof reflected unmodulated radiation on the reflected modulated radiationto provide an amplitude modulated beam of radiation focused on thedetector element.
 6. A system for optically coupling components of oneor more integrated circuits as claimed in claim 5 wherein the reflectivemeans is partially covered by the modulator component, the focussingmeans being located relative to the uncovered area of the reflectivecoating to focus a part of the electromagnetic radiation from the sourceonto the uncovered area whereby unmodulated radiation reflected from theuncovered are is superimposed on the reflected modulated radiation bythe focussing means to provide the amplitude modulated beam.
 7. A systemfor optically coupling components of one or more integrated circuits asclaimed in claim 1 wherein the focussing means comprises a lens and apartially reflecting mirror.
 8. A system for optically couplingcomponents of one or more integrated circuits as claimed in claim 1wherein the focussing means comprises a lens and an arrangement ofmirrors.
 9. A system for optically coupling components of one or moreintegrated circuits as claimed in claim 1 wherein the focussing meanscomprises a lens and a hologram component.
 10. A system for opticallycoupling components of one or more integrated circuits as claimed inclaim 9 wherein the hologram component is in the form of a modulatablehologram thereby providing a switching function within the system.